Ion source



Jan. 18, 1955 5, LUCE 2,700,107

ION SOURCE Filed Jan. 10. @950 INVENTOR- John 6'. Auc'e ATTOPNE Y2,700,107 Patented Jan. 18, 1955 fiice ION SOURCE John S. Luce, OakRidge, Tenn., assignor, by mesne assignments, to the United States ofAmerica as represented by the United States Atomic Energy CommissionApplication January 10, 1950, Serial No. 137,714

4 Claims. (Cl. 250-413) The present invention relates to ion sources,and especially to an improved arc chamber for utilization in massspectrometers, isotope separators, and the like.

Mass spectrometers and spectrographs are well-known in the art. See, forexample, The Particles of Modern Physics, I. D. Stranathan, TheBlakiston Company, pages 154-211. An isotope separator commonly called acalutron, is discussed in Atomic Energy for Military Purposes, H. D.Smyth, Princeton University Press, 1946, pp. 187-205. For a descriptionof use of the calutron in the concentration of stable isotopes, seeChemical and Engineering News 25, 2624- (1947), and The Production ofStable Isotopes and Their Uses in Research, AECD-2436 (November 18,1948). In such apparatus, ions are produced in an arc chamber bybombarding vaporized atoms with electrons from a heated filament. Theresulting positive ions are accelerated out of the chamber by anelectrical potential, are bent through an are by a magnetic field atright angles to the electric field, and are collected at a targetmember, or receiver.

The efiiciency of such devices is greatly influenced by the design ofthe arc chamber wherein the ions are generated. Since the ion collectionat the target obviously can be no greater than the ion production at thesource, it is apparent that the efiiciency of the source in ionizing thecharge material is a criterion of first importance in the operation ofthis class of devices. Likewise of prime importance is the prevention ofrecombination of the ions with electrons to form a neutral atom, foronly charged atoms are focussed onto the target by the apparatus. Eachrecombined atom is either ionized a second time or lost as a neutral gasmolecule before it enters the ion beam; hence, it can be seen that eachrecombination lowers the efficiency of the apparatus.

Ion sources of varied design are known to the art. Generally, theycomprise a chamber to contain the gas or vapor charge, an inlet passagefor admitting the charge material, a filament for emitting electrons, acollimating slot maintained at a potential more positive than thefilament for accelerating the electrons across the chamber and definingthe beam, an ion exit passage, and slits aligned therewith foraccelerating positive ions out of the chamber. A typical massspectrometer source is illustrated in Review of Scientific Instruments11, 213. While sources of these conventional types have proved adequatefor many applications, their efficiency has been uniformly poor; thatis, the number of positive ions leaving the chamber is substantiallyless than the number of atoms of charge material admitted per unit time.ThlS low efiiciency is evidenced by a high rate of charge consumption,and excessive rate of deposit of ions and neutralized ions upon thewalls and accelerating slits, the last named result causing sparking,arcing over, and often complete failure of operation of the apparatus.

It is apparent that in equipment for separating macroscopic quantitiesof certain isotopes, where the charge material is very costly, andseparation rate, or output, 1s of prime importance, an improvement whichresults in increased eificiency of the source is greatly to be desired.Accordingly, it is a primary object of my invention to provide an ionsource so designed as to increase the efliciency of mass separators,mass spectrographs, and the like. 7

Another object ofmy invention is to provide an improved ion sourcecharacterized by a beam of oscillating electrons so designed as toobtain greater ionizing efficiency of the charge material therein and todecrease the rate of deposit of ions and molecules on the walls of thesource.

A further object of my invention is to provide an ion source whereinrecombination of ions is substantially reduced by means of a specialelectron drain arrangement.

Yet another object of my invention is to provide an arc chamber of suchdesign as to support and maintain a more uniform arc.

Other objects and advantages of my invention will become apparent fromthe following detailed description of a preferred embodiment thereof,when read in conjunction with the accompanying drawings, in which:

Fig. 1 is a sectional view of an ion source known in the prior art;

Fig. 2 is a sectional view of my improved ion source;

Fig. 3 is a fragmental perspective view of an embodiment of my improvedsource; and

Fig. 4 is an end view of my improved source showing a preferredalignment of the two defining slots therein.

Referring now to Fig. 1, a horseshoe-shaped tantalum cathode 1 ismounted a short distance from a graphite box 2, defining an arc chamber,which serves as an anode. A small defining slot 3 cut in the chamberwall opposite the cathode 1 permits electrons to enter the chamber andmove within a beam defined by the slot 3 along the length of thechamber. A slot 4 is cut in the back wall of box 2, from a point nearthe chamber wall opposite the defining slot 3 to a point substantiallythe length of the chamber from the defining slot, to admit the vaporizedcharge material, contained in a charge bottle mounted in an ovenadjacent the arc chamber. An opening 5 is provided in the front of thebox 2, extending substantially the full length of the chamber to permitions to escape from the chamber. The chamber may, for example, be 8long, 1%" wide, and 1 /2" deep, while the slit 5 may be 7 wide and 7 /2long.

As shown in Fig. 2, one embodiment of my improved ion source includes ahorseshoe-shaped cathode 9, which may be formed from tantalum Wire .17inch in diameter, and which may be connected to a current source of 200400 amperes to heat the wire to a temperature of approximately 2000 C.in the manner of operation known to the art. Shield 10 prevents escapeof electrons given off by the filament, and tends to focus the electronsback toward the arc chamber in a manner known to the art. Shield 11 is aconventional thermal shield, and also prevents arcing from ground tofilament. The are chamber may be defined by the end walls 17, 18 andupper and lower walls 19, 20 of the rectangular carbon member 14, andmay, for example, be 8" long, 1%" wide, and deep. In the end wall 18 iscontained a slot 15, which may be in the elliptical shape more clearlyillustrated in Figs. 3 and 4, for admission of electrons into thechamber, while in wall 17 is contained a similar slot 30, alsoillustrated in Figs. 3 and 4. Over the open front face of the member 14is fitted a slotted carbon member 21. The slot in that member may extendsubstantially the length of the chamber, and may be inch wide, inaccordance with the principles known to the art. A boxlike carbon member8 is fitted to the back of member 14 and is joined thereto andmaintained in proper alignment therewith by means of protuberance 22which is: press-fitted into groove 23 in wall 18. The chamber and member8 may, of course, be cut from a single carbon block to form an integralunit, if preferred. In the front face of member 3 is cut vapor entryslot 16, which preferably extends from wall 24 to a point substantiallythe length of the chamber, as shown in the sectional view of wall 25.The rear face of the member 8 communicates with a source of vapor, notshown. Anode 34, which may be a stepped carbon plate of any convenientthickness, may be mounted to wall 25 and insulated therefrom by threadedinsulator 35, and may extend parallel to wall .17 at any convenientdistance therefrom, substantially to the front end thereof. An intervalof is one convenient clearance distance which has been employed. Screw36 extends through the body of the insulator and through wall 25, and issecured in position by nut 33, holding.

the insulator firmly to the wall.. Screw 32 fastens the anode 34 to theinsulator 35, without touching screw 36 or wall 25.

It is to be understood, of course, th at the members 8 and 14 may beconstructed from a single block of carbon or other material known to theart, and that the chamber walls may be curved or straight withoutdeparting from the spirit or the scope of the present invention, ofwhich the foregoing is but a preferred embodiment.

Referring now to Fig. 3, which illustrates a preferred construction ofthe anode and cathode collimating slots: both slots are formed byelliptical arcs, rounded off with a circular are at each end. Thecathode slot is preferably .094 in width throughout, while the anodeslot, indicated by the dotted lines, is preferably .062" wide. As isapparent from the drawing, the anode slot is aligned centrally of thecathode slot, so that an electron beam defined by the cathode slot andpassing through the anode slot to the anode would cause the electrons inthe front and back .016" of the beam to drain onto the Wall 17. p

The improvements which I have made in ion sources can be best understoodfrom a description of the operation of the embodiment illustrated inFig. 2 when employed in a calutron mass separator of the type describedby Smyth, supra, and a brief discussion of some of the problems anddifficulties associated with operation of prior sources.

Vaporized molecules of the charge material, which may, for example, beUCli, enter the chamber of memher 8' from the heated charge container,pass through vapor slot 16, and enter the are chamber. There, many ofthese molecules are ionized by the arc flowing from heated filament 9,through cathode defining slot 15, through anode defining. slot 30, toanode 34. Many of the ionized atoms are accelerated out of the chamberbythe action of an electrostatic field set up between the chamber andsuccessive accelerating electrodes. For example, the chamber may operateat ground potential, the first accelerating electrode, not shown, may be40,000 volts negative, and the second successive electrode, not shown,may be 30,000 volts negative. Filament 9 also is maintained negativewith respect to the walls of the arc chamber by, for example, 100 volts.As is known in the art, the pressure in the arc chamber may bemaintained very low, of the order of one to five microns, and thetemperature of the source may be maintained at 500-600" C.

Because the entire assembly is located in a strong magnetic field, as isfully set forth by Smyth, supra, electrons emitted from the filamenttend to travel along magnetic lines of force. Due to the potentialdifference existing between the filament and the arc chamber walls,electrons travel in helical paths parallel to the magnetic field throughthe slot 15, which defines the beam, across the chamber, and out throughslot 30 to impinge upon the anode. If the insulated anode be leftfloating, then electrons will collect thereon and build up a negativecharge sufficient to repel many electrons back through the slot 30 andacross the chamber. At the filament, those electrons will again berepelled across the chamber to the anode, and that oscillatory movementwill continue until the electron combines with an ion to form r amolecule or strikes a wall. of the chamber and is drained off. it hasbeen recognized that by setting elec trons into oscillation back andforth across the chamber, the effective ionizing efficiency of a sourceshould be increased, because where in a source like that of Fig. 1, eachelectron makes only one pass through the vapor. In a source adapted toproduce oscillations electrons may make several such passes, and theprobability that a given electron'will strike a vapor molecule isincreased. No substantial increase in efliciencydue to such oscillationshas been achieved, however, which warranted incorporation of suchfeatures in standard sources.

However, in combination with the above means for providing anoscillating electron motion across the cham her, I have found that bycritically positioning a defining slot near the anode I can markedlyimprove the ionizing efficiency of the ion source. The anode slot ismade uniformly narrower than the cathode slot so that only the centralportion of the electron are reaches the anode,

the outer portion being drained off by the edges of the anode slot. ihaveob'se'rved the greatest improvement when about one-third of the beamis so removed, but noticeable improvements occur when other amounts ofthe arc intercepted. The removal of the outer portion of the are sets upa positive space charge on the outer surface thereof, causing an absenceof electrons about the arc plasma, which includes the oscillatingelectrons and ions of the charge material in a sharply defined region ofhigh ion density. Because of this removal of electrons by the edges ofthe anode slot, the recombination of ions with electrons issubstantially reduced; and as a result thereof, a greater number of ionsmay be drawn out of the are plasma to form the ion beam. it is apparentthat every recombination requires another lectron collision to reionizethat molecule, so that preventing recombination of ions already formedand in the arc plasma, ready to be accelerated to the collectorelectrode, will effectively increase the apparent efiiciency ofionization of the source.

Moreover, arcs in prior sources were subject to instability caused bythe rapid drain of positive ions from the plasma by the highaccelerating voltages applied to the slit electrodes, leaving behind anexcess of electrons. My new and novel electron draining anode slotremoves a great number of these excess electrons. In addition, it hadpreviously been observed that positive ions will not leave the arcplasma as rapidly as is desired, because of the electrostatic forceexerted by the electrons in the plasma to retain them therein. But mynovel drain arrangement removes many of those undesired electrons, andallows rapid exodus of the desired positive ions.

A further improvement in operation of the ion source has been effectedby moving the vapor entry slot, through which the vapor enters the arcchamber, towards the filament end of the chamber from its normalposition shown in Fig. 1. This improvement is evidencedby a decrease inbuild-up of a deposit of neutralized uranium ions, for example, whereUCL; is the charge material employed, at the anode end of the chamber.That deposit results in part from unequal vapor distribution within thearc chamber, the chlorine ions tending to accumulate near the filamentend of the chamber. Because the ionization potential of chlorine is ofthe order of 35 volts, while that of uranium is only about 3 volts, andbecause the electrons lose energy by collision as they travel across thechamber, at the anode end of the chamber more uranium will be ionizedthan chlorine. But at the filament end of the chamber, where electronenergies are high, much of the chlorine will also be ionized. To securemore uniform ion distribution, therefore, vapor is admitted at the highenergy end of the chamber, and blocked oif near the anode end. Adecrease in formation of metallic deposits on the source which causeshort circuits, block free exit of ions, and tend to close definingslots, results from my redistribution of vapor.

An additional improvement tending to overcome the problem of impropervapor distribution, and consequently providing a more uniform arcplasma, results from the structure of my improved source" in admittingthe vapor to the arc chamber very close to the are. In a preferredembodiment of my ion source, the vapor may be admitted substantiallyfrom the arc plasma, as compared to double that distance in priorsources. In the prior deep boxes, the vapor tended to distribute itselfunequally, as was pointed out above, and even though the vapor wasadmitted close to the filament end of the chamber, some improperdistribution would occur during the passage of the vapor from the entryslot to the arc in the front of the chamber. But by introducing thevapor very close to the arc, so that the molecules enter the are almostimmediately upon entering the chamber, improper self-distribution withinthe chamber is minimized, and the desired uniform are results.

While my invention has been described in connection with a specificembodiment and application, it is to be understood that the novelfeatures of source construction set forth herein are not limited'to thecalutron, but apply equally to mass spectrometers or spe'ctrograp'hs ofthe more conventional type, to spectrometers having crossed electric andmagnetic fields, and to other known ion separators and particleaccelerators. Nor is my invention restricted to the separation of theisotopes of uranium, that element havingbeen utilized for illustrationonly, and not to define the limits of the invention.

E claim:

l. in an ion source for the'ionizing of atoms of -a gaseous materialadmitted theretoeby bombardment of said atoms with an electriclarc', afilamentfor emitting a copious suppl'y'of electrons, a source ofelectrical power connected thereto for heating said filament, anelectrode electrically insulated from said filament disposed indirective relation with said filament, a shallow conductivewalledchamber disposed between said filament and said electrode and having anaperture in opposite side walls thereof for passage of said are betweensaid filament and said electrode, means for establishing a magneticfield across said chamber parallel to said are a source of electricalpotential connected both to said filament and said chamber formaintaining said chamber at a more positive potential than said filamentfor accelerating electrons into said chamber, the back wall of saidchamber having a passage therein for admission of said gaseous material,one end of said passage being located at the filament end of saidchamber, said passage being substantially shorter than said chamber, thefront wall of said chamber having therein a passage for exit of ions,said aperture in said side wall nearest said electrode being disposed soas to intercept portions of the front and back of said arc, therebyremoving electrons from said source and deterring recombination of ionstherein.

2. An ion source comprising in combination a shallow arc chamber forcontaining a gaseous material to be ionized, the back and front walls ofsaid chamber each containing therein a passage for the admission of saidsubstance and the exit of ions, respectively, one end of said admissionpassage being located at the filament end of said chamber, saidadmission passage being substantially shorter than said chamber, afilament and an electrically insulated anode electrode disposed atopposite sides of said chamber external thereto, a source of filamentcurrent, a source of potential connected between said filament and saidchamber to accelerate electrons from said filament into said chamber,the side walls of said chamber each containing an elliptical aperturefor defining a beam of electrons emitted by said filament, means forestablishing a magnetic field across said chamber parallel to said beamsaid aperture nearest said anode being uniformly narrower than thataperture nearest said filament to intercept portions of said beamnearest the back and front of said chamber for preventing recombina tionof ions with electrons in the vicinity of said beam.

3. In an ion source including a filament, a source of filament current,means for admitting gas to be ionized, and means for removing ions, afirst beam defining member having an aperture therein, means foraccelerating electrons through said aperture an arc chamber theimprovement comprising an electrode disposed in directive relation withsaid filament, said first beam defining member, and said arc chamber ofsaid source, and means for establishing a magnetic field across saidchamber parallel to said filament electrode alignment said electrodebeing electrically insulated from the remainder of said source, wherebyelectrons are induced to oscillate through said chamber between saidelectrode and said filament, and a second beam defining member disposedbetween said first member and said electrode, and including a slotsubstantially conforming in contour to said aperture in said firstmember but of a lesser cross-section than said aperture and disposed incritical alignment therewith.

4. In an ion source for the ionizing of atoms of a gaseous materialadmitted thereto by bombardment of said atoms with an electric arc, afilament for emitting a copious supply of electrons, a source ofelectrical power connected thereto for heating said filament, anelectrode electrically insulated from said filament disposed indirective relation with said filament, a conductive-walled chamberdisposed between said filament and said electrode and having an aperturein opposite side walls thereof for passage of said are between saidfilament and said electrode, means for establishing a magnetic fieldacross said chamber parallel to said arc a source of electricalpotential connected both to said filament and said chamber formaintaining said chamber at a more positive potential than said filamentfor accelerating electrons into said chamber, the back wall of saidchamber having a passage therein for admission of said gaseous material,the front wall of said chamber having therein a passage for exit ofions, said aperture in said side Wall nearest said electrode beingdisposed so as to intercept portions of the front and back of said arc,thereby removing electrons from said source and deterring recombinationof. ions therein.

References Cited in the file of this patent UNITED STATES PATENTS2,470,745 Schlesman May 17, 1949 2,489,344 Washburn Nov. 29, 19492,511,728 Long June 13, 1950

